All proposals operate under the assumption that the blade enchantment maintains the 25% chance on hit to proc...
Proposal 1: Strike your target for X spirit damage (reasonable value might lie somewhere between 300-600 (100-250 on AoE abilities)) This would operate somewhat similarly to the BW buff (Flames of Rhuin) but would be self only and so the values should reflect that.
Proposal 2: The proc-ing strike deals 50% more damage, converted to spirit damage if it wasn't a spirit attack. Additionally, the Sword Master's defensive target is healed for half the damage dealt.
Proposal 3: The proc provides a 5s buff that is consumed on the SM's next strike. Next strike is undefendable (parry/block are ignored) and heals all groupmates within 30ft of the SM for the value of the damage dealt.
Proposal 4: Remove a single hex/ailment/curse/taunt/challenge from the SM or the SM's defensive target. The SM gains a brief burst of speed (20%?) for 2s if Heaven's Blade proc is unable to remove a detrimental effect. (ICD of a minimum 4s on this one. AoE unable to proc this effect except for a proc check on primary target)
Proposal 5: The proc provides a 10s buff on the SM. The SM receives 5% less damage from all sources, receives 10% more healing from all sources, and the SM lashes out at anyone dealing a critical blow to the SM for 20% of the damaging attack (before the SM's defenses reduced the damage) as spirit damage.
Proposal 6: Heaven's Blade Debuff placed on an enemy refreshes all dots currently on them to full duration. All damage dealt from dots (from all sources) are amplified by 10% on a debuffed enemy. Heaven's Blade has a 10s duration, and can not be refreshed by the SM, but another SM could place their own HB debuff on the enemy, refreshing all dots once more, and adding an additional 10% amplification effect.